This invention relates to connections to coaxial cables and especially to splices between coaxial cables.
In particular the invention relates to the formation of sealed electrical connections by means of dimensionally heat-recoverable article.
Heat-recoverable articles are articles the dimensional configuration of which may be made substantially to change when subjected to heat treatment.
Usually these articles recover, on heating, towards an original shape from which they have previously been deformed but the term "heat-shrinkable", as used herein, also includes an article which, on heating, adopts a new configuration, even if it has not been previously deformed.
In their most common form, such articles comprise a heat-shrinkable sleeve made from a polymeric material exhibiting the property of elastic or plastic memory as described, for example, in U.S. Pat. Nos. 2,027,962; 3,086,242 and 3,597,372. As is made clear in, for example, U.S. Pat. No. 2,027,962, the original dimensionally heat-stable form may be a transient form in a continuous process in which, for example, an extruded tube is expanded, whilst hot, to a dimensionally heat-unstable form but, in other applications, a preformed dimensionally heat-stable article is deformed to a dimensionally heat-unstable form in a separate stage.
In the production of heat-recoverable articles, the polymeric material may be cross-linked at any stage in the production of the article that will enhance the desired dimensional recoverability. One manner of producing a heat-recoverable article comprises shaping the polymeric material into the desired heat-stable form, subsequently cross-linking the polymeric material, heating the article to a temperature above the crystalline melting point or, for amorphous materials the softening point, as the case may be, of the polymer, deforming the article and cooling the article whilst in the deformed state so that the deformed state of the article is retained. In use, since the deformed state of the article is heat-unstable, application of heat will cause the article to assume its original heat-stable shape.
Numerous methods of forming splices between coaxial cables have been proposed. One form of splice that has proved to be particularly successful is that described in U.S. Pat. No. 4,144,404, the disclosure of which is incorporated herein by reference. In this form of device a single in-line splice can be formed between a pair of coaxial cables by means of an arrangement that includes a connector for joining the central conductors of the coaxial cables, and a heat-shrinkable sleeve that contains a shield portion, for example a solder-impregnated braid. In order to form a splice, the heat-shrinkable sleeve containing the shield portion is slipped over one of the coaxial cables and then the central conductors of the cables are connected by means of the connector, for example in the form of a small heat-shrinkable sleeve or ferrule that is provided with one or more solder rings. After the central conductors have been connected, the heat-shrinkable sleeve containing the shield portion is slipped over the splice region so that each end of the shield portion overlaps part of the adjacent coaxial cable shield, and the sleeve is heated to cause it to recover about the cables and to cause the shield portion to contact each coaxial cable shield.
Although this form of coaxial cable splice has been very satisfactory in practice it suffers from a number of problems: For example it is not possible to form splices between coaxial cables, and especially so-called "branch-off" splices in which a single cable enters one end of the splice and two or more cables exit from the opposite end of the splice, that will maintain their integrity when subjected to large changes in ambient pressure as may be experienced for example in an aircraft as it repeatedly changes altitude. It is possible in certain cases for the recoverable sleeve to fracture at the end of the shield portion when the splice is subjected to a bending stress, or for the edge of the shield portion to pierce the outer heat-shrinkable tubing. In addition, it is possible for microchannels to be formed between the interior of the splice and the exterior of the splice along the coaxial cable by ejection of solder and/or flux before the heat-shrinkable sleeve has been able to cool.